The Octave on a note panel is the mode of vibration that occurs when the note panel is divided across its Long Axis. The term is used to refer to this mode of vibration, which may or may not be tuned to an actual octave above the Fundamental. The job of a tuner is to force this mode to be exactly an octave above the Fundamental. It’s common to use the term in ways such as “The Octave is sharp” or “The Octave is flat”, meaning that this mode is sharp or flat.